1. Technical Field
The present invention relates to an exterior enclosure for housing a portable information processor, such as a notebook-size computer and a personal digital assistance, and to a method of manufacturing the enclosure.
2. Background Art
Recently, improvements have been made in providing lightweight portable information processors, such as a notebook-size computer and a personal digital assistance. Under these circumstances, a lightweight and strong material such as magnesium alloy has come to be widely used in producing the exterior enclosure of a display section incorporating therein a liquid crystal display device. Further, in the manufacture of such a exterior enclosure, a method of press forming of plate material capable of producing thinner walled products to achieve reduction in weight has come to be adopted rather than a method of manufacturing them by melt-shaping, such as magnesium die-casting.
FIG. 4 is a sectional view of the display section of a conventional portable information processor. This drawing illustrates a structure based on the design values. Enclosure back panel 31 covers liquid crystal display device 32 from behind. Enclosure back panel 31 has elevated portion 31A raised along ridge lines 31B. Elevated portion 31A has its surface formed flat to provide top plane 31C. In front of liquid crystal display device 32, there is disposed enclosure front panel 33.
Such a structure as described above is called a bonnet structure. The bonnet structure is used when fabricating an enclosure with a thin-walled material. When fabricated in a bonnet structure, the strength of enclosure back panel 31 is enhanced. An example in which such a bonnet structure is applied to an enclosure of a portable information processor is disclosed, for example, in Unexamined Japanese Patent Publication No. 2003-204174.
Fabrication sequence of enclosure back panel 31 in the press forming process will be described below. FIG. 5 is a schematic diagram showing a process for fabricating enclosure back panel 31 by the press forming method. Plate-shaped blank material 44 of magnesium alloy is sandwiched between blank holder 46 and die set 43 and pressed down by punch 42, in a condition heated by heater 45.
At the time when elevated portion 31A is formed by press forming, however, a material distortion occurs around ridge line 31B. Accordingly, elevated portion 31A is deviated from flatness and deformed wavy. In cases where the material is magnesium alloy, it is generally practiced to perform the press forming operation at temperatures above 200° C. to prevent cracks from being formed due to bending the material. During the course of this heating, thermal expansion occurs in the material. Therefore, when a material as above is used, volume of the material becomes more than that of necessity in particular, and hence elevated portion 31A is deformed into a sagged and swelled shape. FIG. 6 is a sectional view of enclosure back panel 31 in which such a shape is represented with exaggerated dimensions for the sake of explanation. The difference between the convexed and concaved levels in actuality is only around ±0.5 mm. However, when the surface is deformed to have irregular concavity and convexity, it impairs the appearance. In addition, the convexed portion is easily dented when subjected to a slight external force. Thus, the surface loses its tautness and its dent resistance is significantly deteriorated.